The invention relates to catalysts for the disproportionation of aryl- or alkylhalodisilane into aryl- or alkylhalomonosilane and aryl- or alkylhalopolysilane, and also to a process for the disproportionation of aryl- or alkylhalodisilanes using these catalysts.
Aryl- and alkylhalosilanes, i.e. silanes substituted by aryl or alkyl groups and halogen atoms, in particular methylchlorosilanes, are valuable industrial starting materials for producing silicone products.
Methylchlorosilanes such as dichlorodimethylsilane and trichloromethylsilane are prepared, inter alia, by the Mueller-Rochow synthesis. However, this produces, after distillation of the products, relatively high-boiling residues in amounts of up to 10% by weight. From these distillation residues there can be obtained a fraction which boils in the temperature range of about 150.degree. C. to 160.degree. C. and which essentially comprises alkylhalodisilanes, such as methylchlorodisilanes. From both economic and ecological points of view it is desirable to be able to work up the distillation residues and thus achieve complete utilization of the raw materials used. Previously known processes for catalytic disproportionation of, for example, methylchlorodisilanes into monomeric methylchlorosilanes and methylchloropolysilanes preferably use as catalytically active substances
amines, for example NR.sub.3 where R.dbd.H, alkyl, aryl, and also the corresponding quaternary ammonium salts PA1 quaternary phosphonium salts PA1 hexamethylphosphoramide (HMPA) PA1 cyanides, in particular silver cyanide. PA1 H PA1 alkyl, branched or linear, preferably C.sub.1 to C.sub.6 -alkyl PA1 oxygen PA1 halogen PA1 trialkoxysilyl PA1 NR.sub.2, in which R.dbd.H, C.sub.1 to C.sub.6 -alkyl, linear or branched, or trialkoxysilyl or be bonded in the ring via 2 sigma and 1 .pi. bond or can form the bridgehead to a further ring. PA1 H PA1 alkyl, branched or linear, preferably C.sub.1 to C.sub.6 -alkyl PA1 halogen PA1 oxygen PA1 NR.sub.2, in which R.dbd.H, C.sub.1 to C.sub.6 -alkyl, linear or branched, or trialkoxysilyl
Disadvantages of these catalytically active substances include, for example, a low activity or the fact that long reaction times are required, which results in polysilanes which, despite the long reaction times, often have a very syrupy consistency, which can be attributed to a high proportion of relatively low molecular-weight polysilanes and which makes their further processing difficult and leads to low yields on conversion into ceramic products. In addition, the proportion and the composition of the monosilane fraction obtained can only be varied within narrow limits.
On the other hand, for example, the use of HMPA cannot be approved without question because of its physiologically questionable nature, despite its good catalytic properties. Despite the efforts of the prior art, there remains a need for improved catalysts for disproportionating aryl- and/or alkylhalodisilanes to monosilanes and polysilanes.